In U.S. Pat. No. 5,020,091 there is described a cellular telephone communications system that includes multiple cells covering a geographical area, such as a metropolitan area. To the base station of each cell a number of frequencies are assigned, with some of the assigned frequencies being specified as control channels used for reciprocal identification between the telephones and the system. The location of the dedicated control channels in the cell's frequency spectrum identifies the type of cellular system, A or B (see, for example, EIA/TIA-553). A metropolitan area typically has one of each type of system.
In a cellular system of most interest to this invention the channel from a base station to the mobile station is referred to as the forward channel, i.e., the forward control channel and the forward voice channel. The channel from the mobile station to the base station is referred to as the reverse channel, i.e., the reverse control channel and the reverse voice channel.
The mobile station's transmitting channels are, by example, from 824.040 to 848.970 MHz and the receiving channels are, by example, from 869.040 MHz to 893.970 MHz. Further by example, the frequency 844.980 MHz is channel 666 for the mobile station's transmitter, and the frequency 889.980 MHz is channel 666 for the mobile station's receiver. When a system allocates a voice channel to the mobile station (e.g., channel 666), it allocates a frequency pair for receiving and transmitting. The frequency pair is referred to simply as channel 666. The receiving and transmitting frequencies of a given channel are always spaced apart by 45 MHz from one other.
Voice channels and access channels use both the forward and reverse channels. The system may have one or more forward control channels used for paging purposes only, and the mobile station is not allowed to transmit on the corresponding reverse channel (i.e., 45 MHz below the paging channel frequency). The forward paging channel is used only for receiving pages and orders from the base station.
However, in most cases the access channel set and the paging channel set are the same, and may comprise 21 channels (i.e., 42 frequencies). In this case the mobile station is allowed to transmit on the reverse control channels, while the corresponding forward control channels are used for paging and also for sending orders to the mobile station. In this case the orders can also include messages from the base station to the mobile station, which are related to system access, e.g. registration confirmation or initial voice channel designation (in the case of a page response).
In greater detail, and referring to FIG. 5A, there is shown an exemplary mobile station transmit (TX or reverse) frequency band and an exemplary mobile station receive (RX or forward) frequency band. The depiction of FIG. 5A is based on the EIA/TIA-553 frequency band allocation (mandatory). Each band is divided into 666 frequency channels, with half of the frequency channels being assigned to the A system and the remaining half to the B system. Although not illustrated in FIG. 5A, it should be realized that additional or extended frequency bands, also described in EIA/TIA-553, could also be allocated. The total width of the mandatory band is 20 MHz+20 MHz.
Each band includes a set of dedicated control channels (e.g., 21 control channels) for the A system, and a corresponding set of dedicated control channels for the B system. The dedicated control channels are used to send information to the mobile station so that it can proceed to the paging channel selection task. The dedicated control channel set is identical across North America (i.e., channels 334-354 for the B system and channels 333-313 for the A system).
In the illustrated case there is a 45 MHz separation between the TX and RX channels. That is, the TX channel number 1 is 45 MHz below the RX channel 1, the TX channel number 2 is 45 MHz below the RX channel 2, etc. As such, it is useful to think of frequency channel pairs, wherein a given mobile station, when assigned to a voice channel (e.g., channel 22), would receive from the base station at a frequency of n MHz, and would transmit to the base station at a frequency of (n-45) MHz. For the dedicated control channels, and by example, dedicated control channel 1 of the B system (i.e., channel 334) could be received by the mobile station from the base station at frequency n, and the mobile station may transmit to the base station at frequency (n-45) MHz.
In most if not all current systems each dedicated control channel is also a combined paging and access (P/A) channel, wherein paging messages and system access information are transmitted to the mobile station on a forward dedicated control channel, and wherein the mobile station transmits access requests (e.g., call origination messages) and other information to the base station on the corresponding reverse channel of the P/A channel pair.
The base stations frequently transmit an Overhead Message Train (OMT). The OMT includes a System Parameter Overhead Message (SPOM) that consists of two words. The SPOM always contains System Identification (SID) information. The SID is unique for each cellular system. The use of the SID enables the cellular phone to determine whether it is using its home system or whether it is roaming.
According to current standards (e.g., EIA/TIA-553), the SPOM is transmitted every 800.+-.300 ms. The OMT may contain additional words, such as the REGID, which in some systems is sent in, by example, every tenth OMT.
As is depicted in FIG. 1, a typical cellular network is divided into a number of regions (e.g., SID1-SID5) each having contiguous radio cells (shown nominally as hexagons). One region or SID may also include one or more paging areas, each encompassing one or more radio cells.
Referring to FIG. 2, when the mobile station is powered on, or after a re-scan timer has expired, the mobile station enters a task where it scans the dedicated control channel set, determines the received signal strength indicator (RSSI) for each channel, and tunes to the strongest channel. The mobile station must receive a SPOM message within three seconds of tuning to the strongest control channel. If the mobile station is not able to complete this task, it either tunes to the second strongest channel, or it continues the channel selection process until the mobile station is able to lock onto one of the dedicated control channels.
Each cell site (or sector) usually has one control channel and several voice channels. Control channels and voice channels are selected according to the network frequency plan, but they include the channels 333 and 334 which, as is shown in FIG. 5B, are adjacent channels of different cellular systems (i.e., System A and System B).
A problem that is addressed by the teaching of this invention results from the use of a 10 kbits/second continuous data stream on the forward control channel. The spectrum of this signal is wider than the channel spacing (30 kHz), which means that some of the signal's energy appears also on adjacent channels (one channel above or below the channel). In the example of FIG. 5B, energy from channel 334 (System B) is shown as leaking into channel 333 (System A).
As was indicated above, when the cellular phone is powered on, performs a re-scan, or accesses the system, it first selects the serving system. If System A is selected, the cellular phone scans channels 333 . . . 313, measures the RSSI of each channel, and tunes to the best channel (strongest RSSI). The cellular phone must then receive message words within predetermined period of time before it can lock onto the channel and/or proceed to other tasks.
The problem arises when the cellular phone selects, by example, System A, and the cellular phone is located too near to a cell site which belongs to System B and which has the control channel of 334. Because of the presence of a large signal level on channel 334, the channel scan and RSSI measurement task can inadvertently determine that the adjacent channel 333 is the strongest control channel in the System A, when actually channel 331 (for example) is the strongest control channel. In this case the cellular phone may tune to channel 333 and then go to the Idle mode or task. The problem is that channel 333 is not a valid channel, at this location, for serving system A. Instead, the cellular phone is actually monitoring the "ghost" of the System B control channel 334. Since the cellular phone cannot normally receive any orders (page messages etc.) from the System B (assuming that System A is the phone's serving system), the cellular phone is essentially cut-off from service in its home system.
Problems also exist with call originations and terminations, since the cellular phone is normally not allowed to make calls in the System B when registered in System A (and vice-versa).
Another problem is that the cellular phone may illuminate its roaming indicator, even though the user expects that he or she is actually in the home system. A system selection feature, such as "A-only", is of no help, since the cellular phone remains tuned to the control channel set associated with the desired system.